1. Technical Field of the Invention
This invention relates generally to communications and more particularly to timing circuits used to support cable, satellite, and wireless communications.
2. Description of Related Art
Communication systems are known to support wireless and wireline communications between wireless and/or wireline communication devices. Such communication systems include, for example, national and/or international cellular telephone systems, satellite, cable television, the Internet, point-to-point in-home wireless networks and radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, 3GPP, LTE, LTE Advanced, RFID, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof. There are three main television standards used throughout the world—NTSC (National Television Standards Committee), SECAM (Système Électronique pour Couleur avec Mèmoire) and PAL (Phase Alternating Line).
Depending on the type of communication system, for example a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID reader, RFID tag, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to one or more antennas (e.g., MIMO) and may include one or more low noise amplifiers, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier(s) receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Currently, wireless communications occur within licensed or unlicensed frequency spectrums. For example, wireless local area network (WLAN) communications occur within the unlicensed Industrial, Scientific, and Medical (ISM) frequency spectrum of 900 MHz, 2.4 GHz, and 5 GHz. While the ISM frequency spectrum is unlicensed there are restrictions on power, modulation techniques, and antenna gain.
The oscillations needed in such wireless circuits as well as those needed in systems such as set top boxes and satellite tuners, must be very precise, with high reliability, and very low noise. A crystal oscillator (XTAL oscillator) is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. Most are used for consumer devices such as radios, computers, television tuners, set-top boxes, and cellphones.
Local oscillator signals used in cellular telecommunications applications must be highly stable. Since crystals, such as quartz, have an extremely high Q, crystal oscillators are often utilized to provide the necessary frequency stability. Typically, quartz crystals are cut and mounted to vibrate best at a desired resonant frequency or an overtone (multiple) of the desired resonant frequency. When the crystal is vibrating, the crystal can be modeled as an RLC circuit that produces a rapidly changing reactance with frequency, with the RLC circuit providing positive feedback and gain at the resonant frequency, leading to sustained oscillations.
Typically, the regular current mirror for XTAL biasing has lower signal-to-noise ratios (SNRs) due to 1/f noise, which cannot achieve better than −130 dBc/Hz spot phase noise (PN) at 1 KHz. Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities (“jitter”). Known use of an RC filter to filter the 1/f noise is very expensive with long startup times as the corner frequency is set to less than 1 KHz. Alternately, inverter based XTAL oscillators may have excessive loop gain variation over process-voltage-temperature (PVT) to produce an oscillation condition.
Disadvantages of conventional approaches will be evident to one skilled in the art when presented in the disclosure that follows.